I. Introduction
This invention relates to electroplating nonconductors, and more particularly, to processes for electroplating the surface of a nonconductor having a semiconductive coating over its surface. The invention is especially useful in the manufacture of printed circuit boards by a process involving passage of the boards through an apparatus, preferably in a horizontal mode.
II. Description of the Prior Art
Nonconducting surfaces are conventionally metallized by a sequence of steps comprising catalysis of the surface of a nonconductor to render the same catalytic to electroless metal deposition followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Metal plating continues for a time sufficient to form a metal deposit of the desired thickness. Following electroless metal deposition, the electroless metal deposit is optionally enhanced by electrodeposition of a metal over the electroless metal coating to a desired thickness. Electrolytic deposition is possible because the electroless metal deposit serves as a conductive coating that permits electroplating.
Catalyst compositions useful for electroless metal plating are known in the art and disclosed in numerous publications including U.S. Pat. No. 3,011,920, incorporated herein by reference. The catalyst of this patent consists of an aqueous suspension of a tin-noble or precious (catalytic) metal colloid.
Electroless plating solutions are aqueous solutions containing both a dissolved metal and a reducing agent in solution. The presence of the dissolved metal and reducing agent together in solution results in plate-out of metal in contact with a catalytic metal tin catalyst. However, the presence of the dissolved metal and reducing agent together in solution can also result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. This may necessitate interruption of the plating operation, removal of the plating solution from the tank and cleaning of tank walls and bottoms by means of an etching operation. Indiscriminate deposition may be avoided by careful control of the plating solution during use and by use of stabilizers in solution which inhibit indiscriminate deposition, but which also retard plating rate.
Attempts have been made in the past to avoid the use of an electroless plating solution by a direct plating process whereby a metal is deposited directly over a treated nonconductive surface. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of the nonconductive surface with a tin-palladium colloid which forms an essentially nonconductive film of colloidal palladium particles over the nonconductive surface. This is essentially the same tin-palladium colloid used as a plating catalyst for electroless metal deposition. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation and growth from a conductive surface. Therefore, deposition begins at the interface of a conductive surface and the catalyzed nonconductive surface. The deposit grows along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow. Moreover, deposit thickness is uneven with the thickest deposit occurring at the interface with the conductive surface and the thinnest deposit occurring at a point most remote from the interface.
An improvement in the process of U.S. Pat. No. 3,099,608 is said to be described in U.K. Patent No. 2,123,036 B, incorporated herein by reference. In accordance with the process described in this patent, a surface is provided with metallic sites and the surface is then electroplated from an electroplating solution containing an additive that is said to inhibit deposition of metal on the metal surface formed by plating without inhibiting deposition on the metallic sites over the nonconductive surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconductive surface in a solution of a tin-palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from a group of dyes, surfactants, chelating agents, brighteners and leveling agents. Many of such materials are conventional additives for electroplating solutions.
There are limitations to the above process. Both the processes of the U.S. and U.K. patents for electroplating require conductive surfaces for initiation and propagation of the electroplated metal deposit. For this reason, the processes are limited in their application to metal plating solutions of nonconductive substrates in areas in close proximity to a conductive surface.
One commercial application of the process of the U.K. patent is the metallization of the walls of through-holes in the manufacture of double-sided printed circuit boards by a process known in the art as panel plating. In this application, the starting material is a printed circuit board substrate clad on both of its surfaces with copper. Holes are drilled through the printed circuit substrate at desired locations. To provide conductivity, the hole walls are catalyzed with a tin-palladium colloid to form the required metal sites on the walls of the through-holes. Since the circuit board material is clad on both of its surfaces with copper and the circuit board base material is of limited thickness, the copper cladding on the surfaces of the circuit board material is separated by the thin cross section of the substrate material. The next step in the process is direct electroplating over the catalyzed hole walls. Since the copper cladding on each surface is separated by the cross section of the substrate, during electroplating, deposition initiates at the interfaces of the copper cladding and the through-hole walls and rapidly propagates into the holes. The hole wall is plated to a desired thickness within a reasonable period of time. Thereafter, the circuit board is finished by imaging and etching operations.
A disadvantage to the above panel plating process is that copper is electroplated over the hole walls and over the entire surface of the copper cladding. The steps following plating involve imaging with an organic coating to form a circuit pattern and removal of copper by etching. Therefore, copper is first electrolytically deposited and then removed by etching, a sequence of steps which is wasteful of plating metal, etchant and time, and therefore, expensive.
The art, recognizing the disadvantage of panel plating, has developed a method for manufacturing printed circuit boards known as pattern plating. In this process, a printed circuit board base material is drilled at desired locations to form through-holes. Through holes are metallized using conventional electroless plating techniques. Electroless copper is plated onto the walls of the through-holes and over the copper cladding. Thereafter, photoresists are applied and imaged to form the circuit pattern. The board is then electroplated with copper depositing on the copper conductors and through-hole walls, but not over the entire surface of the copper cladding. Soldermask is then plated over the exposed copper by immersion or by electroplating and the remaining photoresist is stripped from the surface. The copper not protected by the solder is then removed by etching to form the copper circuit.
Pattern plating cannot be used with the metallizing process of the aforesaid U.K. patent. The treatment of the copper cladding prior to the application of the photoresist and the development of the photoresist, all as required for pattern plating, requires the use of treatment chemicals found to dissolve or desorb the tin-palladium colloid from the hole walls. Since this occurs prior to electroplating, direct electroplating to provide conductor through-holes becomes impossible.
Further improvements in the processes for the direct electroplating of nonconductors are disclosed in U.S. Pat. Nos. 4,895,739; 4,919,768 and 4,952,286, all incorporated herein by reference. In accordance with the processes of these patents, an electroless plating catalyst, such as that disclosed in the aforesaid U.K. patent, is treated with an aqueous solution of a chalcogen, such as a sulfur solution, to convert the catalytic surface to a chalcogenide surface. By conversion of the surface to the chalcogen conversion coating, the electroless plating catalyst does not desorb from the surface during metallization, and consequently, in accordance with the processes of said patents, it is possible to pattern plate substrates in the formation of printed circuit boards.
The processes of the aforementioned patents provide a substantial improvement over the process of the U.K. Patent. However, it has also been found that treatment of an absorbed catalytic metal over a nonconductor with a solution of a chalcogenide, especially a sulfide solution, results in a formation of a chalcogenide on all metal surfaces in contact with the solution of the chalcogen. Therefore, if the process is used in the manufacture of printed circuit boards, the copper cladding or conductors of the printed circuit board base material are converted to the chalcogenide together with the catalytic metal. If the chalcogenide of the copper is not removed prior to plating, it can reduce the bond strength between the copper and a subsequently deposited metal over the copper.
An alternative direct plate process is disclosed in U.S. Pat. No. 5,108,786 incorporated herein by reference. In accordance with the process of this patent, the surface of a substrate is pretreated with an electroless plating catalyst of the type described above and then, following acceleration, treated with a reducing solution such as a solution of a borohydride or an amine borane. Treatment with the solution of the reducing agent is said to improve the resistance of the so formed semiconductive coating to subsequent treatment chemicals. In practice, it has been found that treatment with the solution of the reducing agent results in deposit formation on the copper cladding of circuit board materials which interferes with copper-to-copper bond strength.
An additional alternative direct plate process is disclosed in U.S. Pat. No. 5,071,517. Again, an electroless plating catalyst is adsorbed onto the surface of a part to be plated. In accordance with the process of this invention, the catalyst used is modified by the introduction of an aromatic aldehyde into the catalyst formulation during its make-up. The aromatic aldehyde is said to increase the chemical resistance of the semiconductive layer. Though not disclosed in the patent, but practiced in commerce, the next step in the process involves treatment with an accelerator to which a soluble copper salt is added. The soluble copper salt is said to increase the rate of deposition from an electroplating solution. It has been found in practice that the use of this accelerator also results in formation of a coating on the copper cladding on circuit board materials.
In the manufacture of printed circuit boards by either conventional electroless procedures, or by direct plate procedures, it was the practice of the industry to use vertical processing techniques. Vertical processing comprises conveyorizing the circuit boards in a horizontal path and vertically lowering the boards into treatment tanks for chemical processing. More recently, the industry has moved from vertical processing to horizontal processing techniques as such horizontal processing techniques provide advantages over vertical processing. These advantages include reduced manual handling due to an ability for continuous production flow from one process step to another; simpler mechanics and easier automation procedures; an ability to process boards of differing sizes and thickness; individual processing of boards with more consistent results; vertical orientation of through-holes permitting easier cleaning; reduced immersion time due to forced flood and suction resulting in efficient through-hole solution flows; processing in an enclosed environment reducing operator exposure to chemical fumes and solutions; and the use of pinch rollers to reduce drag out of processing solutions. Though the advantages of horizontal processing were known, difficulty was encountered in attempting to apply horizontal techniques to direct plate processes.